1. Field of the Invention
This invention relates to multi-code signaling, and more specifically to compact representation of multi-code signaling in communication systems.
2. Discussion of the Related Art
For some communication systems, WCDMA (Wideband Code Division Multiple Access) is used as the access technique. In order to improve the data rate for a single user, this user may be allowed to utilize several spreading codes (multi-code operation). One specific area, where this multi-code operation is utilized is in HSDPA (High Speed Downlink Packet Access), where the user's spreading factor is set to the fixed number of 16.
For HSDPA, a base station needs to signal to the mobile node or user equipment (UE) exactly how many multi-codes have been allocated and at which offset the set of codes begin (all defined at spreading factor 16 level). It is assumed that only clusters of consecutive codes are allocated to one user at a time.
In general, up to 15 multi-codes may be supported by the most capable UEs and for the single-code case there is up to 15 different code offsets possible since a single code (offset 0) is reserved to the common channels P-CPICH/P-CCPCH. Commonly, 4 bits are used in order to represent number of multi-codes and 4 bits are used in order to represent the code tree offset. Hence, altogether 2×4=8 bits are needed to make full-flexibility signaling.
Some have proposed that only a subset of multi-code allocations should be allowed thereby enabling a representation by fewer bits, e.g., down to 5 bits. However, since more flexibility with respect to multi-code utilization leads to a better spectral efficiency as well as higher RRM flexibility (QoS, code/time multiplexing), it is desirable to only limit the possible multi-code set as little as possible; preferably not at all.
Some combinations of multi-code number and code offsets are not possible. For example, if 15 multi-codes are allocated to a user there is only one possible offset combination. In total there is only 120 combinations. By building a lookup table with the possible combinations, 120 combinations can be represented using 7 bits. However, the major drawback of the lookup table method is that it requires additional memory at the UE side to decode the received information.
Some have also proposed that instead of indicating the starting position and the number of codes, that the code assignment information is only reported among the possible combinations of code allocation as an alternative means of preserving full flexibility while reducing the number of bits. However, this method leads to conditioned signaling which should be avoided when possible.
It has also been suggested to reduce the number of possible multi-codes such that for instance 1, 5, 10 and 15 multi-codes are the only options that are available. This solution will require 6 bits for the signaling, but at the cost of less flexibility in the code allocation.
Therefore a need exists for reducing the signaling overhead using bit-efficient ways of signaling while maintaining performance and flexibility.